This invention relates to a method and apparatus for adjusting the installation position of switch members in a numerical control system and, more particularly, to a method and apparatus for adjusting the position at which a zero-point return dog or deceleration limit switch or the like is installed in a numerically controlled machine tool.
In a numerically controlled machine tool of the type which uses a device such as a pulse coder or resolver as a position sensor, a so-called zero-point return dog and a deceleration limit switch are provided in order to return a movable machine member, such as a table, to a zero point. The zero-point return dog is installed on the movable member at a predetermined position, and the deceleration limit switch is installed on a stationary portion of the machine tool at a point close to the zero point. Such an arrangement is disclosed in, for example, U.S. Pat. No. 4,112,291 entitled "Positioning system in a numerically controlled apparatus", issued on Sept. 5, 1978. When the numerical control device employed in the system of the type described is placed in the zero-point return mode, the movable element is quick-fed toward the zero point until the zero-point return dog, carried on the movable member, presses against the deceleration limit switch. At the instant this occurs, the feed rate of the movable element is reduced so that the movable element may approach the zero point at a slower speed. Then, after a predetermined period of time has passed, the dog separates from the limit switch which is therefore allowed to return to its former state. When this occurs, the numerical control device, regarding the initial grid point as the zero point, stops the movable element at this grid point to end the zero-point return operation. The grid points mentioned here are a plurality of points, spaced apart by a pitch L, starting with the zero point. The pitch L ordinarily is two millimeters.
It can be understood from the above that the movable element will not be stopped exactly at the zero point if the installation position of the deceleration limit switch or of the zero-point return dog happens to be shifted in excess of one grid pitch. For example, assume that there is a shift in the installation position of the zero-point return dog so that the deceleration limit switch is restored to its original state between a grid point G.sub.1 which is one short of the zero point G.sub.0, and a grid point G.sub.2 which is two short of the zero point. This will cause the numerical control device to regard the grid point G.sub.1 as the zero point, so that the movable element will be stopped at G.sub.1. As an attempted solution, one might install the dog or the limit switch in such a manner that the limit switch will be restored to its former state somewhere between the zero point G.sub.0 and the grid point G.sub.1. However, if the position at which limit switch restoration is achieved is very close to either G.sub.0 or G.sub.1, a slight change in the contact between the dog and the limit switch, or a change with the passage of time, may shift the limit switch restoration position to a point outside the region defined between G.sub.0 and G.sub.1. This would make it impossible to achieve a correct zero-point return. A problem of the type described occurs quite often because of the short distance, on the order of two millimeters, between grid points.
It is obvious from the foregoing that the position at which the deceleration limit switch or the zero-point return dog is installed must be so adjusted that the limit switch is restored to its original state at a position approximately midway between the zero point G.sub.0 and the grid point G.sub.1. It has been conventional practice for the operator to effect such adjustment by a trial and error method in which the operator relies entirely upon his own judgement in mounting the limit switch and dog on the machine and then executes a zero-point return operation to confirm whether it can be accomplished correctly. If not, the switch and/or dog must be remounted. Obviously, this is a complicated and time consuming procedure. In addition, since the position for limit switch restoration will in some cases be extremely close to either the zero point G.sub.0 or grid point G.sub.1, it is quite likely that a correct zero-point return operation will eventually become impossible for the reason given above.